1. Field of the Invention
The present invention relates to a scanning optical apparatus in which a light flux from a light source is transmitted through a deflector and a lens system and scans on a scanning surface and, more particularly, to a scanning optical apparatus having a function to detect and correct a focal position deviation of an image forming spot of the light flux on the scanning surface which is caused due to an environmental fluctuation of temperature or the like.
2. Related Background Art
In recent, years, as a scanning optical apparatus, there has been wide use of a laser beam printer apparatus in which a laser light source is modulated in accordance with an image signal, a laser beam from the modulated laser light source is periodically deflected by a deflector, the laser beam is focused like a spot onto a photosensitive recording medium as a scanning surface by the lens system, and the light spot is exposed and scanned, thereby recording an image.
However, the conventional laser beam printer apparatus has a drawback such that each of the components constructing the lens system is thermally deformed by a change in environmental temperature, an image forming position of a laser beam on the photosensitive material (scanning surface) is deviated, and the picture quality deteriorates.
As means for solving the above drawback, there has been disclosed a construction in JP-A-60-100111, which provides temperature detecting means to measure a temperature of a collimator lens to collimate a laser beam from a laser light source is arranged near the collimator lens, a signal from the temperature detecting means is compared with a preset reference temperature, and a lens moving apparatus to move the collimator lens in the optical axis direction in accordance with a comparison output. The collimator lens is moved in the non-image region in the scanning line in correspondence to an output signal indicative of the non-image region.
However, in the case of actually constructing the laser beam printer apparatus as disclosed in JP-A-60-100111, there occurs a situation such that the collimator lens is moved by the signal indicative of the non-image region even during the operation in which the laser light source is modulated in accordance with an image signal and an image is recorded onto the photosensitive material (scanning surface). Therefore, there is a drawback such that the size and contrast of the pixel change on different scanning lines during the image forming operation and the output image quality cannot be made uniform.